Prostate Cancer
Prostate cancer is cancer that starts in the prostate gland. It is the third most common cause of death from cancer in men of all ages and is the most common cause of death from cancer in men over age 75. Prostate cancer is rarely found in men younger than 40. Systemic chemotherapy is often ineffective in the treatment of prostate-confined cancer. In-vivo results from regional delivery of chemotherapy in prostate cancer (1) show potential higher efficiency in tumor arrest but still have major issues to be solved. Chemotherapy treatment requires large drug amounts, having toxicity comparable to the recommended dose of 60 to 80 mg/m2 for intravenous administration. One general cause of anticancer drug resistance is the limited ability of drugs to penetrate tumor tissue and to reach all of the tumor cells in a potentially lethal concentration (2). Extravasation and interstitial transport (via diffusion and convection) are diminished in the intratumoral space by high interstitial pressure, hypovascularity, high tumor cell density and/or a large stroma fraction; these problems are more serious in larger, bulky tumors. Chemotherapy drugs are more effective against proliferating vs. quiescent cells; thus, slowly proliferating cells at greater distances from tumor blood vessels are likely to be resistant to therapy. Chemotherapeutic drugs also are typically inefficient against tumor stem cells.
Prostate-specific antigen (PSA) is a protein produced by cells of the prostate gland, whose level is reported as nanograms of PSA per milliliter (ng/mL) in the blood. While a PSA level below 4.0 ng/mL was previously considered normal, one large study showed the presence of prostate cancer in 15.2 percent of men with a PSA level of ≦4.0 ng/mL (2), 15% percent of whom (approximately 2.3 percent overall) had high-grade cancers. In another study, only 25-35 percent of men with PSA level between 4.1-9.9 ng/mL and who underwent a prostate biopsy had prostate cancer. Thus, there is no specific normal or abnormal PSA level, particularly since factors such as inflammation (e.g., prostatitis) and variation between laboratories can cause a PSA level fluctuations. In general, however, higher PSA levels correlate with higher probabilities of cancer.
The Gleason grading system is widely used in prostate cancer. It is determined by summing a primary (representing the majority of tumor) and secondary (assigned to the minority of the tumor) Gleason grade, each a number between 1 and 5. The sum of the two patterns is the Gleason score, which has prognostic significance. Patients with a Gleason score of ≦4 do well clinically, while patients with a score of 8-9 do poorly. A Gleason score of 6 typically is followed by “watchful waiting”.
RNA Interference
Non-coding RNAi molecules regulate genes post-transcriptionally and can lead to gene silencing. Endogenous dsRNA initiates RNAi by activating the ribonuclease protein Dicer, which binds and cleaves double-stranded RNAs (dsRNAs) to produce double-stranded fragments of 20-25 base pairs with a 2-nucleotide overhang at the 3′ end, known as siRNAs. These interfering RNAs (siRNAs) are integrated into an active RNA-induced silencing complex (RISC), while being separated into single “sense” and “antisense” strands. Within the RISC, the antisense strand then base-pairs to its target mRNA and induces cleavage of the mRNA, thereby preventing it from being used as a translation template. Synthetic siRNA can vary widely in their design, including the specific sequence along the mRNA, accessibility to Dicer and RISC, the length of each strand, optional symmetrical, asymmetrical, blunt, and loop structures, and chemical modifications of many types.
The delivery of RNAi to target tissue is a major challenge. Systemic injection of siRNA into the vascular system needs to overcome renal filtration and phagocytosis and degradation in the bloodstream, and needs to achieve targeting to the diseased site, transport across the vascular endothelial barrier, diffusion through the extracellular matrix, uptake into the cell, escape from the endosome, and unpackaging and releasing the siRNA to the cell RNAi machinery. Systemic delivery today is limited to a small number of target tissues, in particular to the liver.
Even direct injection of naked siRNA to topical targets (for example the eye, skin, mucus membranes, and localized tumors) and intranasal/intratracheal instillation of aerosolized siRNA into the lung is subject to rapid dose decline by diffusion and degradation and increased pressure (in some cases of injection). Repeated injections at a frequency of about one per week are often required.
Alshamsan et al. (STAT3 Silencing in Dendritic Cells by siRNA Polyplexes Encapsulated in PLGA Nanoparticles for the Modulation of Anticancer Immune Response, Molecular Pharmaceutics 7(5): 1643-1654, 2010) reported nanoparticles containing siRNA complexed with polyethylenimine (PEI). However, these devices exhibit fast drug release, typically on the order of one week, and are ineffective to carry high drug loads to a wide tissue area, for a sufficient treatment period.
US Patent Publication No. US2008/0124370 (Marx) describes reagents, methods and systems to treat inflammation and pain in a subject using small interfering RNA (siRNA) molecules targeted to either TNF-alpha, Il1, IL6 and other pro-inflammatory cytokines.
US Patent Publication No. US 2011/0195123 (Shemi) describes an implantable medical device eluting drug locally and for a prolonged period, treatment methods, and implantation methods. The device comprises a polymeric substrate and a drug, for example gene silencing drugs based on RNA interference (RNAi), including siRNA, shRNA, or antisense RNA/DNA, ribozyme and nucleoside analogs.
Thus, a continuing need exists for RNAi compositions to effectively treat prostate cancer.